The Marcellus Shale, a member of the Devonian Black Shales, is the largest known deposit of natural gas in the United States. It extends from Ohio and West Virginia, through Pennsylvania and into Southern New York, an area of 54,000 square miles, which is several times the size of the Barnett Shale. The Marcellus Shale has the potential to fulfill America's natural gas needs for years to come.
Hydraulic fracturing is a process whereby fluid under pressure is used to fracture open the shale, such as that in the Marcellus Shale, to greatly enhance the recovery of natural gas. The fluid used in hydraulic fracturing (often referred to as “fracture water,” “frac water” or “frac fluid”) is primarily water. Other components of frac fluid may include sand proppant, hydrochloric acid, glutaraldehyde as a biocide, sodium chloride, ethylene glycol and various components for scale inhibitor and iron control. The exact composition of frac fluid may vary.
One of the properties of frac fluid is that it is very viscous in order to carry the proppant (sand) into the cracks in the rock. To make the fluid viscous enough, polymers (usually carbohydrates) are added to the frac fluid. Frac fluid unavoidably picks up metals from the deep formations. Some of these metals may be toxic (e.g., arsenic) and/or radioactive (e.g., uranium).
Disposal of the flow-back water (frac fluid) is one of the biggest challenges facing the recovery of natural gas from shale. Each well can produce millions of gallons of flow-back water, because after the fracturing process about 60% of the water used is pushed back up the well by the released gas. Ground water can be protected by casing, but the problem lies in the safe disposal of the flow-back water which can contain arsenic, manganese, cobalt, chromium, lead, various other metals and dissolved wastes, as well as compounds added by the manufacturer. An additional concern is that the frac fluid can be radioactive from Radium 226 and Radium 228.
Current disposal methods are unsatisfactory for environmental or economic reasons and do not provide for the recycling of the frac fluid. These disposal methods include: diluting the fluid with sewer effluent and discharging it into a river or stream or trucking the waste to industrial water treatment facilities.
The methods and systems described in this document are directed to treatment of fracture fluid that is contaminated with heavy metals.